1. The principles of measuring the cut-off wavelength of a single-mode optical fiber by the "bend method" are described below.
The cut-off wavelength is an important parameter of a single-mode optical fiber and means the limit wavelength at which the fiber operates in a single mode without propagating light in more than one mode. It is therefore very important to make precise measurements of this cut-off wavelength. One of the techniques extensively used to determine the cut-off wavelength is called the "bend method" and is shown in FIG. 5.4.29 on page 437 of "Hikari Tsushin Handbook (Handbook of Lightwave Communications)", Asakura Shoten, Sept. 1, 1982. A typical apparatus employed in implementing this method is shown in FIG. 9, where optical fiber 1 of a suitable length whose cut-off wavelength is to be measured is coupled between light source 2 capable of producing varying wavelengths and optical detector 3. Both light source 2 and optical detector 3 are connected to control/calculation unit 4.
The measurement of cut-off wavelength by the bend method proceeds as follows. First, the light issuing from source 2 is swept over a certain range of wavelength, say, 0.95-1.45 .mu.m, under the control of unit 4 with fiber 1 being held in an unbent or slack condition as indicated by the solid line in FIG. 9. Optical power output P1 (.lambda.) from fiber 1 at each of the wavelengths used is detected by detector 3 and stored in unit 4.
Next, fiber 1 is wound around mandrel 5 with a suitable diameter as indicated by dashed line 1A in FIG. 9, with care taken not to cause a change in the state of coupling of fiber 1 to either light source 2 or detector 3. With the equipment set in this way, the light issuing from source 2 is swept over the same range of wavelength as used in the first detection, under the control of unit 4, and optical power output P2 (.lambda.) from fiber 1 at each of the wavelengths used is detected by detector 3 and stored in unit 4.
The ratio between transmission powers P1 and P2 may be calculated as follows: ##EQU1## Based on the data obtained by this calculation, a graph is prepared that shows the wavelengths characteristics of the fiber, or the wavelength dependency of bend loss, as shown in FIG. 10. The cut-off wavelength may be determined from this graph as the rise point of the bend loss.
2. Problems with the measurement of the cut-off wavelength of a single-mode optical fiber by the "bend method" are now described.
The conventional equipment for measuring the cut-off wavelength of a single-mode optical fiber by the "bend method" described above has the following problems in terms of precision and ease of measurement. First of all, the precision of the measurement of the cut-off wavelength is highly sensitive to any change that may occur in the state of coupling of the fiber at both ends when a bend is given to the fiber and great care must be taken to avoid the occurrence of such change. If there occurs any change in the state of coupling of the fiber at either end, the amount of light that enters the optical detector will vary irrespective of the wavelength dependency of the fiber.
For implementing the "bend method" described above, if the fiber is wound around the mandrel so that the fiber fixed at both ends in the "state where no bend is given to the fiber" is put into the "state where a bend is given to the fiber", the fiber inevitably experiences a "torsion" or rotation of the fiber around its central axis. Coated optical fibers generally have a tendency to restore easily from torsion, so the torsion that has occurred in the fiber introduces a change in the state of coupling of the fiber at both ends, thus degrading the precision of measurement of the cut-off wavelength of that fiber.
Furthermore, the coated optical fiber also has a tendency to restore from a bend. Therefore, the fiber simply wound around the mandrel has a rather instable attitude and requires some fixated treatment such as by securing the fiber with adhesive tape. Such treatment, however, not only reduces the ease of measurement, but also causes microbending or other bends that may adversely affect the precision of measurement at the fixed ends of the fiber.
As a further problem, the "bend method" described above has low reliability of measurement because of the low reproducibility and stability of the "state where no bend is given to the fiber." The first reason for this problem is that the meaning of the "state where no bend is given to the fiber" has not so far been clearly defined, and it has been entirely left to the discretion of a specific organizational or individual experimenter to decide as to how the optical fiber should be placed to obtain this state. As a consequence, the "state where no bend is given to the fiber" has differed from one experimenter to another or between runs of measurement. Secondly, with the fiber put in a slack condition as shown in FIG. 9, the slightest change in the state of the fiber will cause a considerable change in the measurement data.
Under these circumstances, the International Telegraph and Telephone Consultative Committee (CCITT) proposed "Revised Version of Recommendation G652 Characteristics of A Single Mode Fiber Cable" in May 1984. On pages 15-18 of this paper, the CCITT proposed that the cut-off wavelength of a single-mode fiber be measured by making comparison of the wavelength dependency between the "state where no bend is given to the fiber" and the "state where a bend is given to the fiber" in accordance with the conventional method except that the respective states are replaced by application of two bends having different radii of curvature, one having a larger radius-of-curvature, say a diameter of 280 mm, and the other having a smaller radius-of-curvature, say, a diameter of 60 mm. Further conditions of measurement that are considered necessary and sufficient by the recommendations are as follows: when a bend with the larger radius-of-curvature is applied, care shall be taken to avoid any bends of radius smaller than the radius selected and the portion to be bent shall be equivalent to a loop having the circumference calculated from the radius selected; and when a bend with the smaller radius-of-curvature is applied, care shall be taken to avoid any bends of radius smaller than the small radius selected and the portion to be bent shall be equivalent to a loop having a circumference not smaller than that calculated from the radius selected.
The method of measurement recommended by the CCITT ensures the high reproducibility and stability of the "state where no bend is given to the fiber", thereby eliminating the possibility of low reliability of measurement that will otherwise occur when no such high reproducibility and stability are ensured.
However, the method recommended by the CCITT still imparts a bend to the fiber under test by the conventional technique of winding the fiber around a mandrel and, so long as this technique is employed, the problems arising from torsion that develops in the fiber and its restoring force cannot be solved. A torsion applied to the fiber under test causes a change in the state of coupling of the fiber at both ends. In addition, because of the force of restoration from a bend applied to a coated fiber, the latter is unable to have a stable attitude if it is simply wound around a mandrel and must be fixed by some means, such as by securing with adhesive tape. This fixing operation requires careful winding of the fiber around a mandrel with a diameter of as large as 280 mm so that the wound fiber will not come loose. In addition, microbending and other bends that will adversely affect the precision of measurement may occur as a result of attachment of the adhesive tape.
With the rapidity of the recent advancement of fiber optics technology, mass production of optical fibers with high quality has become possible. This has required the inspection of fiber products with higher precision and efficiency, and this demand is particularly urgent for single-mode optical fibers with respect to the measurement of cut-off wavelength which is one of the most important parameters of such fibers.
The conventional apparatus for measuring the cut-off wavelength of a single-mode fiber has not been designed in consideration of its size or space factor. The equipment therefore is large in size and the operator finds considerable difficulty in measurement because of the large space that has to be covered during operation of the equipment. In addition, the operator must repeat the winding of the fiber around a mandrel with a suitable diameter for each run of measurement whether the purpose is to produce the "state where no bend is given to the fiber" or the "state where a bend is given to the fiber". In either operation, that portion of the fiber which is not wound around the mandrel must be maintained in a straight or unbent state, and in order to meet this requirement, the operation has to employ a suitable adjusting means such as the moving of junction points so that any insufficiency of the fiber length or sag that will occur as a result of rewinding the fiber around a mandrel with a different diameter may be compensated. However, this may introduce a change in the state of fiber connection at the junction points or a movement of the fiber on either the light source or optical detector side, thereby causing adverse effects on the precision of measurement such as a change in the state of fiber excitation.